1. Field of the Invention
The present invention relates to cavity pressure testing and particularly to HVAC testing of pressurized cavities in an assembly line environment.
2. Discussion of the Background
HVAC testing is widely employed in the manufacture of automotive vehicles which contain numerous vacuum and pressurized cavities throughout the structure of the vehicles, for example, air conditioning systems and engine systems. The efficient production of high-quality vehicles requires that the vacuum or pressurized systems be tested on the assembly line while the vehicles are being assembled. Because both accuracy and speed are required in this environment, manufacturers generally rely on automated test systems.
Among the systems employed by manufacturers for HVAC testing are pressure decay systems, differential pressure decay systems flow meter systems (mass flow rate systems), and totalization of flow (mass flow totalization) systems.
A disadvantage of pressure decay systems is that the cavities or pressure lines to be tested must be first evacuated in order to perform the test, and further, the systems must be tested in the absence of pressurized flow. Pressure decay systems are limited to small cavities and are affected by temperature, pressure, and humidity. Differential pressure decay systems are limited to large cavities and are also affected by temperature, pressure, and humidity. Pressure decay systems report only estimated flow rate.
A disadvantage of the flow meter systems is that the test cycle time tends to be too long to be practical in a modem vehicle assembly line environment, where the target time for testing is approximately 45 seconds. Flow meter systems typically have cycle times of approximately 90 seconds. Mass flow rate systems are affected by humidity, cavity temperature, excessive cycle duration, and are limited to measuring large cavities.
Totalization of flow test systems overcome many of the disadvantages of the above systems. In background art systems which use the totalization of flow principle, a central processor controls test heads which are moved up and down the assembly line and attached to various test points on the vehicles being assembled. Background art test systems utilize test heads that are connected to the central processor via cable connections. A disadvantage, as recognized by the present inventors, of using test heads connected with cables is that moving the test heads from vehicle-to-vehicle is constrained by the length of the cable, and further, the number of test heads that may be attached to a vehicle may be limited because of the difficulty of placing the test heads which are encumbered by the attached cables.
The repeatability of test results of a system investigating a specified cavity is dependent on a number of factors including but not limited to atmospheric pressure, atmospheric temperature, atmospheric humidity, cavity material temperature, and cavity material thermal transfer rate. Using empirical formulas, exact results are achieved when all of these parameters are monitored and included in the methods for determining cavity integrity. However, the level of cost associated with this type of system is inappropriate for typical testing conditions.